Lighting control systems often use daylight harvesting techniques to reduce energy consumption by dimming or turning off artificial lights when natural light is available. A typical daylight harvesting system includes a photocell or other light sensor to measure light in a specific building space. A control circuit adjusts the artificial lighting in an attempt to maintain the total light level at a predetermined setpoint. If the available light, as measured by the light sensor, is at or above the setpoint, no additional light is needed. If the available light falls below the setpoint, the control circuit attempts to turn on just enough artificial light to bring the combined total of natural and artificial light up to the setpoint level.
Daylight harvesting controls typically require a commissioning procedure to configure the controls and adjust various system parameters to operate properly and optimize efficiency. These controls may include inputs that select between open-loop and closed-loop operation, establish the setpoint level, initiate manual or automatic setpoint determination, provide a scaling factor for the signal level of the light sensor, set minimum and maximum output levels for the artificial lighting, and compensate for losses in light output as the sources of artificial light diminish over time. Each of these functions typically has an associated control device such as a switch or dial. For example, a typical daylight harvesting controller may have three or more blocks of DIP switches and several trimming potentiometers to adjust all of these parameters.
Photocells used in daylight harvesting systems typically have a cone-shaped field of view and are often implemented as remote components to facilitate placement in the best location for sensing ambient or task lighting. Some photocells are housed in fixed mountings that are designed to be attached to a building surface, conduit or electrical box. These fixed mountings are sometimes provided with shutters or movable mirrors to adjust the angle or field of view of the photocell. Other photocells are mounted in ball-and-socket assemblies or complicated swivel arms that enable the photocell to be aimed at a particular area of interest. Photocells are also included in lighting control assemblies with motion sensors. The field of view of the photocell and motion sensor are adjusted in unison by aiming the housing at an area of interest.